In recent years, the demand for non-aqueous electrolyte secondary batteries has been increasing as high-energy density batteries that can store electrical energy. Among non-aqueous electrolyte secondary batteries, molten-salt batteries that use flame-retardant molten-salt electrolytes are advantageous in terms of good thermal stability. In particular, sodium molten-salt batteries that use molten-salt electrolytes having sodium ion conductivity can be produced from inexpensive raw materials and thus are regarded as promising next-generation secondary batteries.
Promising molten-salt electrolytes are ionic liquids which are each a salt of an onium cation and a bis(sulfonyl)imide anion (refer to PTL 1). However, the history of the development of ionic liquids is short, and ionic liquids containing various minor components as impurities are used at present. Furthermore, there have been few studies on the effects of impurities on molten-salt batteries, and the effects are in an unexplored area.